1. Field of the Invention
The present invention relates to a die assembly for molding a disc. More particularly, the present invention relates to a die assembly employable when a disc such as a compact disc, a video disc, CD-ROM, MD or the like is molded using a synthetic resin by employing an injection molding apparatus.
2. Background Art
A conventional die assembly for molding a disc using a synthetic resin by employing an injection molding process or an injection/compression molding process is generally constructed such that a mirror surface-finished die board is firmly received in a movable die half. A stamper is placed on the mirror surface die board and this stamper is then immovably held on the movable die half with the aid of an inner ring and outer ring fitted thereto by tightening bolts.
With the conventional die assembly constructed in the above-described manner, when it is required that the present die assembly be replaced with another one, a molding operation is first interrupted, and thereafter, die replacement is achieved while the stationary die half is held still on a molding machine. However, since the die replacing operation must be performed within the narrow space between the opened movable die half and the stationary die half on the molding machine, die replacement is accomplished only at a very low operational efficiency. In addition, there often arises a malfunction so that the expensive mirror surface board is readily damaged due to the limited space therebetween during the die replacing operation. Another malfunction is that foreign matter such as dust or the like is liable to enter the space between the stamper and the mirror surface board, resulting in double reflection, local warpage, projection-shaped flaw or the like being undesirably caused on a molded disc product molded with the conventional die assembly.
In view of the malfunctions as mentioned above, a proposal has been made with respect to a die assembly of the aforementioned type as disclosed in an official gazette of Japanese Unexamined Patent Publication No. 63-303713. According to this prior invention, the die assembly is constructed such that a spacer, a stamper, a mirror surface board, an inner ring and an outer ring are fabricated in the form of an unit component. This enables a die assembly operation to be performed outside of a molding machine without any necessity for replacing the stamper with another one while the die assembly is held still on the molding machine.
With respect to the die assembly constructed according to this aforementioned prior invention, however, since the spacer, the stamper and the mirror surface board are assembled together with the aid of the inner ring and a fixing member fitted to the foremost end part of the inner ring to form a single integrated structure which is then turned together with the inner ring so as to allow the integrated structure to be mounted on the movable die half, there exists a problem that the whole integrated structure must be disassembled after removal from the movable die half and another integrated structure must be rebuilt in place of the former one each time the stamper is replaced with another one.
In addition, for fixing a unit on the movable die half, fixing means disposed at the foremost end of an inner ring must manually be fitted to another fixing means for the whole die assembly by rotating the foregoing unit. The position where the fixing means for the whole die is disposed on the movable die half is inevitably limited to a position in the vicinity of the position where an ejector mechanism required for taking a molded disc product out of the die assembly and a gate cutting mechanism are arranged, and moreover, the fixing means for the whole die must be arranged without any occurrence of interference with the mechanisms as mentioned above, resulting in the movable die half becoming unavoidably complicated in structure.
Since the die unit is inserted from the parting plane side of the movable die half when the present die assembly is replaced with another one, there exists a problem that elaborate care must be taken so that the unit does not collide with each of the aforementioned mechanisms.
Further, a die temperature is one of the significant factors associated with an operation for molding a disc. To assure that double refraction is minimized over the molded disc product and a stamper surface has excellent transferability and dimensional accuracy, the die temperature must be controlled in such a manner that the whole surface of a cavity is kept at a comparatively high temperature which in turn is uniformly and stably maintained for a long period of time.
However, according to the prior invention as mentioned above, the unit can not be equipped with temperature regulating means. Provided that temperature regulating circuits to be incorporated in the die assembly are arranged in a region in the vicinity of a unit fitting surface, they can not practically be arranged at a position near to the surface of a cavity for which temperature regulation is important. For this reason, it has been hitherto considered practically difficult to uniformly and stably control the die temperature at a high level over the whole surface of the die assembly from a remote location. If the foregoing controlling operation is forced to be performed, there probably appears another problem that a temperature regulating unit operable at a high flow rate under a high intensity of pressure is required, resulting in the die assembly being fabricated at an expensive cost.